Preparation of welding surfaces on semiconductors

ABSTRACT

A method of forming a weldable surface on a ceramic member by: 1. DISPERSING PARTICLES OF CUPROUS OXIDE WHICH WILL REACT WITH THE CERAMIC ONTO THE CERAMIC SURFACE; 2. HEATING THE PARTICLES UNDER TIME AND TEMPERATURE CONDITIONS WHICH CAUSE THE PARTICLES TO REACT WITH THE CERAMIC; AND 3. IMMERSING THE CERAMIC IN A NICKEL SOLUTION FOR DEPOSITING A LAYER OF NICKEL ON THE REACTED AREA.

United States Patent Rice, Jr.

[ 1 Mar. 7, 1972 [54] PREPARATION OF WELDING I SURFACES ONSEMICONDUCTORS [72] inventor: Stanley B. Rice, .lr., Garland, Tex,

[73] Assignee: Texas Instruments Incorporated, Dallas,

Tex.

[22] Filed: Apr. 16, 1969 211 App1.No.: 851,507

Related US. Application Data [62] Division of Ser. No. 505,687, Oct. 29,1965, Pat. No.

[52] US. Cl. ..117/212, 117/10 [51] lnt.CI ..B44d l/l8 [58] FieldoiSearch ..117/212, 10

[56] References Cited UNITED STATES PATENTS 3,424,658 1/1969 Norton..117/212 3,296,359 111967 Ramsey, Jr., et a1 ..l17l212 3,259,559 7/1966Schneble, Jr., et 31.. ....1 17/212 2,993,815 7/1961 Treptow ....112/212 7] ABSTRACT A method of fonning a weldable surface on a ceramicmember by:

1. dispersing particles of cuprous oxide which will react with theceramic onto the ceramic surface;

2. heating the particles under time and temperature conditions whichcause the particles to react with the ceramic; and

3. immersing the ceramic in a nickel solution for depositing a layer ofnickel on the reacted area.

2 Claims, 13 Drawing Figures PATENTEDMAR 71912 3,647. 534

SHEET 2 [IF 2 PREPARATION OF WELDING SURFACES ON SEMICONDUCTORS Thisis'a division of copending application, Ser. No. 505,687 filed Oct. 29,1965 now US. Pat. No. 3,494,790.

This invention relates to the preparation on welding surfaces ofsemiconductors, and with regard to certain more specific features toimproved means for obtaining welding surfaces on semiconductor bars orthe like to provide for connecting conductive leads thereto.

Among the several objects of the invention may be noted the provision ofimproved means for forming uniform weldable metalsurfaces on portions ofsemiconductor bars and more particularly on the surfaces of very smallor closely spaced marginal grooves or notches therein; the provision ofa method of fonning such weldable surfaces permitting the advantageoususe of extremely small particles of metal not formerly useable in smallor closely spaced notches or grooves; and the provision of a method ofthe class described which requires less time and smaller quantities ofcertain materials than were heretofore required for forming the desiredweldable surfaces on the ceramic bar material. Other objects andfeatures will be in part apparent and in part pointed out hereinafter.

The invention accordingly comprises the methods, constructions andproducts hereinafter described, the scope of the invention beingindicated in the following claims.

In the accompanying drawings, in which several of various possibleembodiments of the invention are illustrated:

FIG. 1' is a fragmentary perspective view of a ceramic semiconductor barhaving several sidewise notches or grooves in it, one of which isprovided with a weldable surface applied according to the invention;

Smaller particles of aluminum were tried but they did not react with theceramic. This was probably due to the thin film of aluminum oxide, about30 to 40'angstroms thick, which sur- FIG. 2 is an enlarged fragmentarysection through the groove in the FIG. 1 ceramic bar, being viewed online 2-2 of FIG. I;

' FIGS. 3-6 are sections illustrating diagrammatically various stepsemployed in carrying out one form of the invention;

FIG. 7 is a fragmentary cross section taken on line 7-7 of FIG. 2,showing a conventional lead welded to a prepared surface of a notch orgroove;

FIGS. 8-l2 are sections illustrating diagrammatically various stepsemployed in carrying out another form of the invention; and

FIG. 13 is a section similar to FIG. 12 showing a conventional leadwelded to the prepared surface of a notch or groove.

For clarity of illustration, dimensions are exaggerated and thereforethe drawings are not to exact scale.

Corresponding reference characters indicate corresponding partsthroughout the several views of the drawings.

weldable surfaces have been provided on ceramics containing magnesiumoxide and silicate by placing aluminum particles of l to 2 mil size insmall or closely spaced groove therein and then reacting the aluminumwith the ceramic at a temperature of about l,000 C. in an atmosphere ofhelium. After the reaction, excess oxides were scraped from the reactedsurface. The reacted surface was then plated with nickel by anelectroless plating process. Such nickel plating normally occurs at arate of about 0.5 mil per hour, and 3 or 4 mils of nickel are requiredin order to obtain a good weldable surface on the ceramic. Thus severalhours were required for plating this process were not successful, forexample, in grooves 9 mils wide and only 2 or 3 mils deep. Uniformcoverage in the desired metallized areas could not be obtained, probablydue to the inability of the grooves of such small size to containsufficient aluminum powder for intimate contact with the ceramicsubstrate, or in the case of closely spaced grooves, to prevent overlapof conductive material between grooves.

to remove the oxide film was tried but they were found not to I becontrollable. The present invention permits application of a continuous,uniform layer of metal in a very small or closely spaced grooves (orifrequired, on larger areas) and reduces the time required by priormethods for electroless plating of a satisfactory weldable metal surfaceon the ceramic bar.

Briefly, one process for metallizing a ceramic according to thisinvention comprises grinding and mixing together particles of aluminumand another metal (such as tin) having a melting point lower than themelting point of aluminum, then suspending them in a liquid vehicle orcarrier and inserting them by means of a hypodermic needle into a grooveor other surface of the ceramic. Other application means may beemployed. Then the ceramic and metals are heated in a furnace at atemperature to melt tin and aluminum. The metals and ceramic reactduring heating to form a thin continuous metallic film between theceramic and a layer of unreacted oxide on the surface. After removal ofthe oxide, the reacted area is then plated by an electrolessnickel-plating process to provide a nickel surface to which leads or thelike may be welded.

Another embodiment of this invention overcomes the long electrolessplating time required. This embodiment comprises applying powderedcuprous oxide to a ceramic surface, sintering it under conditions whichresult in reducing the cuprous oxide to cupric oxide. Reaction betweenthe cupric oxide and the ceramic forms a good bond with the silicon inthe ceramic throughout a reacted area. Then the ceramic is placed in areduction furnace for removing the oxygen remaining from the cupricoxide. The result is a layer of copper covering the ceramic in thereacted area. This copper layer is pulled off, leaving a very thinmolecular layer of copper on the reacted area. Next the ceramic isplaced in an-electroless nickel-plating solution and plated for about 1hour to deposit about 0.5 mil thickness of nickel on the reacted area.Leads can be welded to the plated area.

Referring now more particularly to FIGS. 1-7 of the drawings, at numeral1 is illustrated a portion of a rectangular ceramic member preferablyformed of a mixture of magnesium and silicate, such as forsterite (2MgO-SiO or steatite (MgO-SiO2). This member, for example, may constitutea so-called silicon or semiconductor bar on which is a semiconductornetwork, requiring leads for forming outside connections. Thus theceramic l constitutes a base or substrate for a so-called integratedcircuit package. There are a plurality of small irregularly shapedmarginal notches or grooves 3 in ceramic member 1 which are to beprepared so that leads can be welded to the package.

Small particles of aluminum, preferably less than 1.8 mils in size, andsmall particles of tin, preferably less than 1.2 mils in size, areground together to smaller sizes. The preferred ratio of tin to aluminumis 1 part tin to 2 parts aluminum by weight. Tin is selected because ofits relatively low melting point (about 232 C. Other metals with lowmelting points can also be used.

The finely powdered metals are then dispersed in a vehicle or carrierwhich holds the metals in suspension while they are being applied to theceramic. The vehicle may be one manufactured by the DuPont company ofWilmington, Delaware and designated in the trade as l i-220. Othervehicles which "can be used-include amyl acetate, dextrim, gum arabic,gum

tragacanth, and other organic substances. The DuPont product ispreferred since it leaves practically no residue after firing. Thevehicleand metal particles suspended by it are applied in the grooves 3in ceramic 1. ln FIG. 3 the metal particles in groove 3 are designatedand the carrier or vehicle is designated 7.

The ceramic 1, carrying the metal particles 5 and carrier 7, is thenplaced in a tube furnace and heated in a reducing atmosphere at atemperature of about 800 C. for about 1 hour. The atmosphere may behydrogen. During heating the tin particles melt at about 232 C. Theyhave a fluxing or dissolving action on the aluminum oxide or thealuminum particles, so as to release the oxide as the aluminum melts atabout 700 C. Then the tin combines with the aluminum particles to form aliquid tin-aluminum alloy. This alloy reacts intimately with themagnesium and silicate of the ceramic so that there is produced acompound or alloy, referred to as spinel (M- gAl,O,). This spinel ispresent in a reacted area or zone designated 9. At the same time thereoccurs a layer or coating 11 of unreacted oxide on the reacted zone. Thereacted area or zone 9 is firmly adhered to the ceramic 1, whereas theunreacted oxide layer 11 is loose and is easily removed. FIG. 5 showsthe ceramic and reacted zone after the oxide layer 11 has been removed.

It may be noted at this point that the process has been tried employingunder the same operating conditions particles of tin or aluminumindividually, instead of together. No reaction occurred in eitherinstance. The tin particles coalesced together in globules on theceramic surface and could be brushed off. The aluminum particles did notreact at all, probably, as above mentioned, because the aluminum couldnot rupture the surrounding film of aluminum oxide to'j'react with theceramic materials. The oxide flushing or diss lving action of the tinremoves this difiiculty.

Next the ceramic and reacted zone or area 9 are placed in a solutioncontaining nickel (or other weldable metal ions) and the reacted zone 9is plated by electroless plating with a layer 13 of nickel. This platingoccurs by a catalytic decomposition of the solution at the surface ofzone 9. A typical "electroless plating solution may contain sodiumhypophosphite. and nickelous chloride with buffers. The reactions whichoccur are as follows:

l-l- P0- +H- iO+HPO +l-I+H (ac id or2H) The hydrogen then reduces thenickel at the surface as follows:

This electroless plating is continued until the desired thickness of thenickel layer 13 is provided. Normally layers of 3 to 4 mils of nickelare required to provide a good weldable metal surface. This platingoccurs at a rate of about 0.5 mils per hour. Thus 6 to 8 hours arerequired to obtain a nickel coating 3 to 4 mils thick. After nickelplating, the nickel is annealed in a conventional manner and leads ofKovar are welded to the metallized surface (see FIGS. 1, 2 and 7).

While the drawings illustrate the nickel layer 13 as being depressed inthe center portion and with the lead 15 located in this depression, itwill be understood that this is only for the purpose of illustration.Also, there may be some small portion of the reacted zone 9 and thenickel layer 13 present on the surface of ceramic 1 adjacent to thegroove 3. If this occurs it can be removed by conventional lapping. Itwill be understood that lowmelting-point metals other than tin may beused for fluxing or dissolving the aluminum oxide from the aluminumparticles.

FIGS. 8-13 illustrate an alternative process of forming a weldable areain a marginal notch, groove or other recess 3 in ceramic 1. In thisembodiment, small particles 17 of copper in the cuprous form (Cu,0) aremixed with a vehicle or carrier 19 in the manner described in connectionwith FIGS. l7. Carrier 19 may be any of the organic substances above,described. The copper and vehicle are applied in the groove 3 (FIG. 8).The ceramic, metal and vehicle are then placed in a 'fumace and sinteredfor about 1 hour in air at a temperature of approximately l,000 C. Thevehicle carrying the copper is 4 the cuprousoxideKCu o) to the cupricform (CuO). The ceramic and sintered copper are cooled, at which timethere exists a crust 21 above the reacted zone 23.

Next the ceramic 1 with the crust 21 on the reacted zone 23 are placedin a reduction furnace for removing the remaining oxygen. After firstflushing the furnace within'ert helium to clear out air for safety,hydrogen is introduced into the furnace at a temperature of about 300 C.This temperature is then raised to about 1,000 C. and held for about 15minutes. The ceramic is then cooled in the hydrogen atmosphere. After ithas cooled and been removed from the hydrogen atmosphere, there occurs alayer 25 of metallic copper covering the reacted zone or area 23 (FIG.10). This layer of copper 25 Y is only loosely adherent and is easilypulled from the reacted zone or area 23 (FIG( 11). The looseness isapparently due to disruptive stresses caused by differential shrinkageupon cooling. When this loose copper layer is stripped sway there isleft on the reacted zone 23 a very thin molecular facing of copper. Thisis indicated by dots 29 in FIGS. 1113. In view of its extreme thinness,this layer is not disrupted upon cooling and therefore remains tightlyon the zone 23. I

The ceramic and the reacted zone 23 on which is the molecular copperlayer are next exposed to a solution containing nickel and, byelectroless plating, a layer of nickel is plated on the reacted zone 23.This nickel layer is designated 27 in FIG. 12. The electroless platingis continued for approximately l hour and a layer of nickel about 0.5mils thick is thus obtained on the reacted area 23. This issubstantially thinner than the 3 to 4 mils thickness usually requiredwith other processes. If desired, the nickel may be annealed before theleads such as 15 are welded to it.

It may be mentioned that the use of cuprous oxide (Cu,O for particles 17is of some importance. An attempt has been made to substitute cupricoxide (CuO) for particles 17 but the result upon reduction in hydrogenwas the formation of copper globules formed on the substrate surfaces,and these were not sufficiently adhered. I

Ceramic articles produced by either method of the invention may be usedin manufacturing integrated semiconductor network packages with leads 15constituting their outside connections. In manufacturing a package ofthis type, an appropriate surface of ceramic 1 is coated with a glassfrit and 4 fired to form a thin glass glaze. Then circuitry isevaporated on the glaze and connected by internal accessory leads to themetallized groove or notch areas, thereby completing a circuit throughthe leads 15.

In view of the above, it will be seen that the several objects of theinvention are achieved and other advantageous results attained.

As variouschanges could be made in the above methods, constructions andproducts without departing from the scope of the invention, it isintended that all matter contained in the above description or shown inthe accompanying drawings shall be interpreted as illustrative and notin a limiting sense.

What is claimed is:

1. The method of forming a weldable metallized surface on a ceramicmember containing magnesium and a silicate, said ceramic member having agroove on a surface thereof, comprising placing cuprous oxide powder insaid groove, sintering the ceramic and cuprous oxide in air at atemperature at about l,000 C. for about 1 hour, cooling the ceramic,cuprous oxide and reactive zone therebetween, heating them in anatmosphere of hydrogen at a temperature of 300 to l,000 C. for about 15minutes and then cooling them in a hydrogen atmosphere to provide asubstantial layer of loosely adherent copper in the groove, removingsaidlayer of copper whereby a molecular layer of copper remains on saidceramic, immersing the ceramic in a solution containing nickel anddepositing by electroless plating a layer of nickel on the molecularcopper layer on the reactive zone.

2. The method of fonning a weldable area on a ceramic member containingmagnesium oxide and a silicate comprising dispersing particles ofcuprous oxide in a vehicle to form a for about 15 minutes and coolingthem in a hydrogen atmosphere to provide a layer of loosely adherentcopper and oxide stripping the loosened mass of copper and oxide fromthe ceramic leaving a molecular layer of copper thereon, and immersingthe ceramic in a solution containing nickel for about 1 hour to depositby electroless plating a layer of nickel on the reacted zone.

0' i t I t in An: In.

2. The method of forming a weldable area on a ceramic member containingmagnesium oxide and a silicate comprising dispersing particles ofcuprous oxide in a vehicle to form a dispersion, applying the dispersionon said area of the ceramic member, sintering the area with the appliedmaterial at a temperature of about 1,000* C. for 1 hour to dissipate thevehicle and react the cuprous oxide with the silicate and to convertsome of the cuprous oxide to cupric oxide, reducing the cupric oxide tocopper, cooling the ceramic, cuprous oxide and the reactive zonetherebetween and then heating them in an atmosphere of hydrogen at atemperature of 300* to 1,000* C. for about 15 minutes and cooling themin a hydrogen atmosphere to provide a layer of loosely adherent copperand oxide stripping the loosened mass of copper and oxide from theceramic leaving a molecular layer of copper thereon, and immersing theceramic in a solution containing nickel for about 1 hour to deposit byelectroless plating a layer of nickel on the reacted zone.
 3. IMMERSINGTHE CERAMIC IN A NICKEL SOLUTION FOR DEPOSITING A LAYER OF NICKEL ON THEREACTED AREA.